Auxiliary power units (“APU”) are used in aircraft to provide electrical power and compressed air to various parts therein. When an aircraft is on the ground, its main source of electrical power and pneumatic power comes from the APU often when the main engines are not operating. In particular, the APU can power the environmental control systems, air drive hydraulic pumps, and the starters for the main engines. When an aircraft is in flight, the APU may also provide pneumatic and/or electric power to the aircraft.
Typically, APUs are located in the aft section of the aircraft, at or near the tailcone section, and utilize an air intake system to allow sufficient air flow to travel from an opening in the exterior surface of the airplane through to the APU. Air intake systems typically include an inlet air door to protect the APU from foreign object damage when not in use and/or during ground movement, and to maximize airflow into the APU when performance at altitude is required. Thus, when APU venting is desired, the inlet air door opens, either on the ground or in flight.
Air intake systems also typically include passageways whereby the air entering through the inlet door flows to the APU for use by the APU. Air intake systems using air inlet plenums are often preferred over traditional duct systems as providing increased APU performance and decreased noise from the APU system.
However, when the air inlet plenum and the opening in the exterior surface of the aircraft are in close proximity to one another (as is often the case with small and medium size aircraft), there are often significant air pressure losses associated with the sudden area changes at the interface.
Therefore, there is a need for an air intake assembly that can deliver air into the plenum while minimizing the pressure losses associated with the sudden area changes.